1. Field of the Invention
My invention relates to apparatus for optically or optoelectronically reading out information, such as video and/or audio information, recorded along a prescribed track on a record carrier such as that in the form of a disc. More specifically my invention deals with an improved focus control in such apparatus for automatically holding the scanning beam of light focused on the information-bearing surface of the record carrier.
2. Description of the Prior Art
Electronics manufacturers the world over have proposed various forms of optical video and/or audio readout apparatus. Among these is the Philips-MCA video disc player. The principle of this well known video disc player applies also to the readout of audio information by the pulse code modulation (PCM) scheme. The record disc for use with such video and/or audio disc players bears information in the form of a succession of short grooves or "pits" arranged along a multiturn spiral track.
Usually, for the readout of the information, the disc player has an optical pickup assembly comprising a light source such as a helium-neon (He-Ne) laser or semiconductor laser, an objective or collector lens for focusing the laser beam on the record disc, and a photodetector for translating into an electrical signal the information-modulated laser beam that has been reflected from the record disc. The rotation of the record disc and the linear motion of the pickup assembly in the radial direction of the record disc enable in combination the scanning laser beam to follow the spiral track on the record disc.
The disc player further includes a focus control for holding the scanning laser beam focused on the record disc. Were it not for the focus control, it would be practically impossible to maintain a constant distance between record disc and objective throughout the course of information readout, because the disc is never perfectly flat.
According to a well known example of focus control system, the laser beam is split into a readout sub-beam and a focus control sub-beam. The focus control sub-beam impinges on the record disc at an angle to its surface and, on reflection therefrom, irradiates a split photodetector consisting essentially of a pair of photodetector elements in close juxtaposition. The boundary between the pair of photodetector elements exactly bisects the spot of the reflected focus control sub-beam falling thereon when the readout sub-beam is focused on the record disc. Thus, as long as the correct distance exists between record disc and objective, the photodetector elements produce electrical outputs of equal magnitude. The output magnitude of either photodetector element becomes greater, and that of the other becomes smaller, when the disc-to-objective distance either increases or decreases from its correct value.
The objective is usually suspended in springs and coupled to a drive coil in a radial magnetic field. The drive coil moves with the objective toward and away from the record disc in response to an energizing current corresponding to the difference between the output magnitudes of the photodetector elements. The objective thus refocuses the readout sub-beam on the record disc.
An objection to this prior art focus control system is that it requires the use of a sub-beam exclusively for focus control, making the disc player unnecessarily complex and expensive of manufacture. Moreover, as changes in ambient temperature put the optical system of the apparatus out of order, the reflected focus control sub-beam may not fall on the split photodetector at the exact boundary between its two component elements even though the readout sub-beam is focused on the record disc. This of course will result in the forced defocusing of the readout sub-beam.
I am aware of several proposed systems that dispense with a focus control sub-beam. Such advanced systems rely on the intensity or spot diameter of the reflected readout beam for detection of the distance between record carrier and objective. My invention also attains the objective of focus control without use of a focus control sub-beam, but through a completely different approach.